1. Field of the Invention
This invention relates to a polarization-maintaining fiber, a specialized optical fiber, polarization optics in a single-mode optical fiber, a fiber gyroscope, fiber-optic interferometers, particularly Sagnac interferometer, fiber-optic sensors, cohererent optical transmission, etc.
2. Description of the Related Art
Guided-wave transmission systems often require that the transmission is in one stable mode. In a conventional single mode optical fiber, because of the possible existence of two degenerate modes, perturbations of various kinds, such as bending, pressure, random twisting, etc., will cause conversions and reconversions of power to occur between the two modes, with the result that light transmission in a selected mode, or in a preferred polarization, becomes annoyingly unstable.
Currently available "polarization maintaining" optical fibers are capable of maintaining a linearly polarized mode only, and only when the orientation of the linear SOP (State of Polarization) is always directed along one of the two principal axes of the fiber. If the linearly polarized light deviates but slightly from the principal axis, the so-called polarization-maintaining fibers no longer maintain the polarization of the propagating light, which actually undergoes all variations of the SOP in a half beat length.
While linear-polarization maintaining optical fibers have achieved admirable successes over the past years, and have found a variety of applications in fiber-optic systems, a major difficulty exists in that all such fibers require alignment of-the birefringence axes of fiber elements in order to be jointed or spliced. In some simpler applications, the aligning of the principal axes can be aided by the technique of light power monitoring, i.e. assuming that two fiber elements are satisfactorily aligned when the transmitted light power attains an optimum value. However, in certain other applications which involve the interference of two or more propagating light beams, the power-monitoring method will not work. One example of this case is provided by a hi-bi (high birefringence) Sagnac fiber loop in a gyroscope architecture, for which the final closing of the loop by splicing the last two fiber ends poses a yet unsolved "hard-nut" problem because of the existence of two counter-propagating beams in the same loop. See Huang Hung-chia, "Passive polarization-controlled all-fiber-gyroscope and other interferometric architectures", Fiber and Integrated Optics, Vol. 12, No. 1, pp. 21-29, (1993).
Birefringent optical fibers which are capable of maintaining a circularly polarized light appear to be an ideal medium for the obvious reason that segments of such fiber can be jointed or spliced easily without the need of a special technique of aligning. In fact, the circular polarization maintaining fibers do not possess such things as the principal axes. Joining or splicing two segments of circular-polarization maintaining fiber is just as simple as the case of conventional optical fibers, for which only the cores on both sides of the joint are required to be aligned.
The advantageous feature possessed by an optical fiber transmitting a stable circularly polarized light was early recognized about one and a half decades ago, when the initial attempts to make linear-polarization maintaining fibers just started. R. Ulrich and A. Simon, "Polarization optics of twisted single mode fibers", Appl. Optics, vol. 18, pp. 2241-2251 (1979). L. Jeunhomme and M. Monerie, "Polarization maintaining single-mode fiber cable design". Electron. Lett. vol. 16, No. 24, pp. 921-922 (1980)
Over the past years, while the art of making linear polarization maintaining fibers has achieved admirable successes in producing several practical fiber versions, notably the PANDA, the BOW-TIE and the elliptical-cladding fibers, assiduous efforts in the search for a practical fiber transmitting circular polarized eigen-modes have not yet made any real advance.
The method early adopted to produce a circular polarized eigen-mode in fiber is to sufficiently twist a fiber of the conventional version. The attempt did not succeed because the necessary twist rate was found to be impractically high, so that the fiber would crack before the twist rate approached the required high value. A. J. Barlow and D. N. Payne, "Polarization-maintainance in circularly birefringent fibers", Electron. Lett., vol. 17, No. 11 pp. 388-389. Primarily for the technical need of a rotation sensor or gyroscope, a special "spin and draw" machine was early devised to draw a circular-polarization maintaining fiber. F. Gauthier et al, "Attempts to draw a circular polarization preserving fiber", Proc. Int. Cong. Fiberoptic Rotation Sensors", MIT, Cambridge, pp. 196-200 (November 1981). Despite all of these efforts, early attempts to fabricate circularly birefringent fibers by spinning the preform resulted in low birefringent spun fibers. Meanwhile, the early need for realizing coherent optical transmission did prompt an experimental study of the feasibility of twisted fiber to transmit a stable circularly polarized light over a comparatively long length of the fiber line. S. Machida, "Polarization preservation in long-length twisted single-mode optical fibers", Trans. IECE of Japan, vol. E65, No. 11, pp. 642-647 (November 1982). While valuable experimental data were obtained through comparatively extensive experimentation, the objective of realizing coherent transmission with the aid of twisted fiber remains unfulfilled.
A more recent approach to secure a circularly polarized eigen-mode in guided light transmission is to make use of, essentially, the geometrical effect of a specialized fiber with a helical core. M. P. Varnham et al, "Design of helical core circularly birefrigent fibers", Proc. OFC, p. 68, Poster Paper TUL 20 (1986). R. D. Birch, "Fabrication and characterisation of circularly birefringent helical fibres", Electron. Lett. vol 23, No. 1, pp. 50-51, (1987). The novelty of the idea of making of a helical-core fiber is attractive, but still, application of such a specialized fiber in a practical system poses a number of problems. Because of the helical path of the core, the diameter or transverse dimension of the fiber is necessarily larger than that of the standard fiber versions, and this is an unfavorable feature in many applications. But more importantly, the special techniques of injecting a beam of light into the helical core, and of joining or splicing segments of such fibers into a line, are by far inconvenient to be used in practice.
But the effort of searching for a fiber of high circular birefringence has never discontinued. In parallel with the helical-core fiber, there evolved in the literature and patent documents a family of so-called circularly form-birefringent fibers whose common working principle is to generate a circular birefringence by way of twisting an intentionally made azimuthal-dependent index distribution of the core. Such fibers are also called twisted "multi-core" fibers, embracing a variety of fiber versions with different index patterns of the core, with the names of "SPIRAL" fiber for one-core lobe, "TWISTED-CROSS" or "CLOVER-LEAF" fiber for four-core lobes, "OCTOPUS" fiber for eight-core lobes, etc. R. Romaniuk and J. Dorosz, Proc. SPIE 403, p. 35, (1983). Y. Fujii and C. D. Hussey, "Design considerations for circularly form-birefringent optical fibers", IEE Proc., vol. 133, Pt.J, No. 4, pp. 249-255 (1986). C. Roberto, "Circularly birefringent optical fibres: new proposals", Opt. & Quantum Electron. vol. 21, pp. 35-46 (1989). C. G. Someda, Italian Patents Numbers 41584A/ 85 (26 Jul. 1985) and 41638A/86 (16 Dec. 1986). Despite their high academic interest, the core index patterns in this family of fibers are so sophisticated that technologically such fibers are difficult, if not impossible, to fabricate. The only published information about an actually fabricated specimen is the one lobe "SPIRAL" fiber, which closely resembles a helix fiber. The multi-core fibers do not appear to be useful from a practical viewpoint, inasmuch as the launching of light into the fiber and the joining or splicing of fiber segments would pose severe problems for the very sophisticated configuration of the fiber-core design.